Our knowledge of the complex system governing the adaptive immune response of vertebrates has grown tremendously in the past few decades. However, we have only begun to realize that many invertebrates, long thought to survive only by primitive means, have also developed impressive defense mechanisms for coping with the hostile elements in their environments. This realization has led to the suspicion that the evolutionary origins of vertebrate immunity may in fact reside amongst the vertebrates. The only way to test this hypothesis is through in-depth studies on selected invertebrate species to determine not only the biological nature of their immune responses, but how the cells and substances mediating these responses compare with the vertebrate counterparts. it is for this reason that we have been carrying out studies on the American cockroach, Periplaneta americana. The advantages of using the roach as an experimental model are the following: The cockroach is relatively long-lived; it is easily maintained in the laboratory; it can be bred in captivity and yield inbred strains of animals that will be invaluable for future genetic studies; and, since we have already demonstrated that the roach possesses an adaptive inducible humoral immune response, we feel it is quite worthy of further study. The studies of Periplaneta include: The direct identification of the factor mediating the humoral response by various protein separation techniques in combination with an in vivo assay; developing a sensitive and quantitative in vitro humoral factor assay; generating specific rabbit polyclonal antibodies against various native hemolymph proteins associated with immune roaches; purifying the humoral factor by the use of ion exchange or affinity chromatography identifying the gene(s) which encodes for the production of the humoral factor; and, identifying which cell type(s) generates the humoral factor. The results from these investigations should provide valuable information not only regarding the definitions of invertebrate immunity, but also for our understanding of the evolution of immune reactivity in general.